This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/SE00/00136 which has an International filing date of Jan. 21, 2000, which designated the United States of America and was published in English.
The present invention relates to a system and a method for microlithographic writing on photosensitive substrates, and specially printing of patterns with extremely high precision, such as photomasks for semiconductor device patterns, display panels, integrated optical devices and electronic interconnect structures. The terms writing and printing should be understood in a broad sense, meaning exposure of photoresist and photographic emulsion, but also the action of light on other light sensitive media such as dry-process paper, by ablation or chemical processes activated by light or heat. Light is not limited to mean visible light, but a wide range of wavelengths from infrared to extreme UV.
A system and method for microlithographic writing of a substrate is previously known from e.g. EP 0 467 076 by the same applicant. In general a system for microlithographic writing, as is shown in FIG. 1, comprises a light source 1, such as a laser, a first lens 2 to contract the light beams, a modulator 3 to produce the desired pattern to be written, the modulator being controlled according to input data, a reflecting mirror 4 to direct the beams towards the substrate 6, and a lens 5 to contract the beams before the reach the substrate. The mirror 4 can be used for a scanning operation to scan the beam over scan lines at the substrate. Several functionally equivalent scanners such as acusto-optic deflectors etc. could also be used. Further, the substrate is preferably arranged on a object table. Two-dimensional relative motion between the lens 5 and the table (stage) is provided and controlled by servo systems. For example the object table could be movable in at least two orthogonal directions, by means of two electrical servomotors. This motion is an inertial mechanical motion, and it is relatively difficult to change the velocity motion quickly. Further, this is irrespective of if it is the object table or the lens that is moving, since both possess a substantial mass.
However, a problem with such known writing systems are that the emitted output power from the laser is not constant, but suffers from significant variations due to so called drop-outs. Such drop-outs are significant drops in the light output, typically at least a 25% power drop, with a typical duration of about 500 xcexcs, and they occurs once every 10 h -1000 h, depending on the type of laser, the age of the laser etc. The problem is especially important for continuous gas lasers.
A short drop-out for about 500 xcexcs can typically result in a loss of about 20 scan-lines in the pattern, and such an unwanted hole in the pattern normally ruins the whole mask. For very large patterns, such as large-area photomasks, which normally take about 10 hours to write, the problem is more likely to occur, at the same time as the value of an error free mask is very high.
Several other problems of the same kind could arise, such as electronic errors, vibration errors and the like. These problems also generate temporary interruptions in the continuous writing process.
Other types of pattern generators encounter similar problems, and the invention is not limited to pattern generators using microlithography.
It is therefore an object of the present invention to provide a system in which the above mentioned temporary abnormal interruption problems in the prior art are solved.
This object is achieved with a system according to the appended claims.
By abnormal writing error conditions are in this application meant error conditions that can be detected and are abnormal in the sense that they could not in practice be foreseen. Hence, problems that occur at regular time intervals are not considered abnormal, and neither is for example xe2x80x9cdata starvexe2x80x9d, i.e. when the supply of data is too slow relative to the writing speed, in a system where this condition can be foreseen from data volume and writing mechanics and handled in an ordinary fashion.